1. Field of the Invention
The present invention relates to an apparatus and a method of inspecting a mura-defect which detect a mura-defect in patterns in a photomask for fabricating an image device, and a method of fabricating a photomask.
2. Description of the Related Art
Conventionally, for a photomask for use in fabricating an image device such as an image pickup device and a display device, mura-defect inspection has been known as one of the inspection items for inspecting the patterns formed on the surface thereof. The mura-defect is an error having different regularities that have been unintentionally generated in patterns regularly arranged, and the defect is generated by some causes during fabrication process steps.
When a mura-defect occurs in patterns in a photomask for use in fabricating an image pickup device and a display device, such a mura-defect is transferred onto the patterns in an image device to cause sensitivity unevenness and display unevenness in the image device, resulting in the performance of the image device being deteriorated.
Further, the mura-defect in patterns of the image device and in patterns of the photomask cannot be detected in pattern inspection for individual patterns in many cases because microdefects are regularly arranged in general. However, when an area is observed as a whole, the defect can be observed differently from the other parts. Therefore, the mura-defect inspection is mainly conducted by visual inspection such as oblique lighting inspection by human eyes.
However, the visual inspection has a problem such that variations are generated in inspection results according to the individual inspectors because visual inspection tends to largely depend on their subjective evaluations. Then, Conventionally, for the image device (for example, a liquid crystal TFT substrate), for example, a mura-defect inspection apparatus described as disclosed in JP 10-300447 is proposed. The mura-defect inspection apparatus according to JP 10-300447 is structured such that light is irradiated onto a substrate formed with patterns on the surface and scattered light from the edge part of the pattern is sensed by a CCD line sensor to detect unevenness.
Another type of a mura-defect inspection apparatus has been also known in which light is irradiated onto a photomask 50 having a repeated pattern where a unit pattern is regularly arranged on a surface 52A of a transparent substrate 52 (FIG. 8) from a light source 62 obliquely downward in a similar way of JP 10-300447, a light receiving member 63 is used for receiving the reflected light from the repeated pattern of the photomask 50 and converts it to received light data, and an analyzer 64 analyzes the received light data to detect a mura-defect generated in the repeated pattern.
In addition, in FIG. 8, a numeral 55 depicts a chip on which the repeated pattern is formed on the surface 52A of the transparent substrate 52 of the photomask 50. Furthermore, over the surface 52A of the transparent substrate 52, a pellicle film 56 is mounted which protects the repeated pattern from dust and dirt with a pellicle frame 57. Moreover, the photomask 50 is placed on a stage 61 as a back side 52B of the transparent substrate 52 in contact with the stage 61.
However, the mura-defect inspection apparatus shown in FIG. 8 raised the following problem which must be taken into the consideration.
First, as shown in FIG. 4(B), the pattern information that a the light receiving member 63 receives for a light 65 includes the pattern information of the scattered light and reflecting at the edge part of the unit pattern in the repeated pattern of the photomask 50. In addition to this, it also receives a light 66 having pattern information that light has passed between unit patterns of the repeated pattern and reflected at a back side 52B of the transparent substrate 52.
Therefore, the analyzer 64 which analyzes received light data from the light receiving member 63 might not detect a mura-defect highly accurately.
Next, as shown in FIG. 5(B), when light is irradiated onto the repeated pattern of the photomask 50 protected by the pellicle film 56 from the light source 62 obliquely downward, an area is generated in the repeated pattern where the light irradiated from the light source 62 is blocked by a pellicle frame 57 and the reflected light cannot be received by the light receiving member 63, and thus a mura-defect might not be detected highly accurately.
Furthermore, as shown in FIG. 6(B), when light from the light source 62 passes through the pellicle film 56, reflects at the edge part of the unit pattern in the repeated pattern, and again passes through the pellicle film 56, the transmittance of the pellicle film 56 affects light intensity to drop to reduce the contrast of received light data at the light receiving member 63, and thus a mura-defect might not be detected highly accurately.
Moreover, as shown in FIG. 7(B), since the stage 61 is in contact with the back side 52B of the transparent substrate 52 and supports the photomask 50, when the transparent substrate 52 has variations in its thickness, the position of the surface 52A formed with the repeated pattern in the transparent substrate 52 is varied with respect to the stage 61. Thus, the focus plane in the light receiving member 63 needs to be adjusted in accordance with the position of the surface 52A of the transparent substrate 52 for each photomask 50.